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4/8 cylinders with low compression

This is a discussion on 4/8 cylinders with low compression within the Internal Engine forums, part of the LSx Technical Help Section category; I have a 2002 Z28 camaro that seemingly runs fine at low speeds but is missing a lot of power ...

  1. #1
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    4/8 cylinders with low compression

    I have a 2002 Z28 camaro that seemingly runs fine at low speeds but is missing a lot of power when I want to get up and go. As I tore into the engine there were quite a few problems. PCV valve needed replacing, O2 sensors needed replacing, and I replaced the intake gasket just in case. After all this and resetting the computer I still had the exact same results. I then started assuming something was going on with the cylinders. I replaced plugs and wires (still no change) and did a compression test. My passenger side cylinders were fine with around 170 psi but my entire driver side was only getting a steady 50-70 psi. At this point I should have done a top-down compression and a wet test but I didn't. My bad. I figured I was going to have to tear into the engine one way or another. My best guess was a strangely bad head gasket or cracks in the block or head. After tearing apart the head I haven't been able to see problems (I am not sure I could see a problem if it was right in front of my nose). The piston walls have zero sign of scoring and I am not sure about the rings. Could they all go bad? The gasket looks okay I guess. The three layers were slightly separated when I took it out and the middle rubber looking layer is extremely hard. Naturally I will replace it but I need some advice. Any thoughts on what is going on?

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    Spaz is My Mentor SMWS6TA's Avatar
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    Compression can be lost by:

    Rings, Head Gasket and Valves or worse case cracked piston.

    It might be worth it to have the head checked.

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    Since the issue seems to be limited to one bank with every hole on that side I'd focus first on the items which would effect the entire bank as opposed to issues that would normally effect individual cylinders (however a prolonged lean condition on one bank could potentially cause a number of issues such as burnt valves, warping and/or cracks on the cylinder head or deck on that side, another possibility, however even less likely could be a restriction in coolant flow to that bank caused by build up in the cooling passages, but you probably would have noticed that upon removing the head) Aside from those unlikely situations I'd be leaning towards the head gasket if everything checks out. Clean/prep the deck and look it over (a magnifying glass is handy to spot small defects, clean the tops of the pistons with what ever cleaning agent you want, personally I use a little 2+2 instant gum cutter or brake clean sprayed on a rag and then apply a thin coat of clean oil with a new rag to the clean area and cylinder walls. Hopefully and most likely you won't find anything. Considering the fact that the other bank checks out it's not likely that the opposing bank would be much different in terms of wear. Next check the surface of the deck with a straight edge to insure it's flat. Also check the mating surface of the head as well. If it checks out turn the head chamber side up and keep it level and with all the valves closed fill each chamber with water in order to check valve seal, they should hold water without issue. If you are still unsure have it checked at a reputable machine shop. If you find an issue with the head it would make the perfect excuse for a new set of heads and a cam swap while you're in there If it turn out to simply be a headgasket I'd suggest doing them both..
    Good luck, post back with what you find!

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    Very odd issue to have an entire bank come in low while the other side tested fine. Did you repeat the test to ensure that your results were accurate? Any chance your gauge was sticking or that the seal came loose on it?

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    Thanks 99Red for the info. I looked at everything you mentioned, put the head back on, and..... nothing. Well, the same steady 50psi of compression I was getting before. Everything seems fine. Valves, piston, block, head, gasket. My main question now is the rings. I was able to put some oil down a cylinder but it had no effect on the compression. Any more ideas??? could the rings be completely gone? Why is it holding the compression? Shouldn't it leak out slowly if there was a problem? Could it be timing or could that only effect the whole car??? Anyone with some ideas would be greatly appreciated.

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    If squirting oil down the plug hole does not raise compression, then its probably not the rings unless they are completely broken out. You indicated that the cylinder walls looked good, so that's not too likely. Only other options are the valves or a crack somewhere in the head. Loosen upon the rockers on the affected cylinder and repeat the compression test. That will ensure that nothing in the valve train is hanging the valves open.

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